1. Field of the Invention
The present invention is directed to a circuit arrangement for serial data transmission between a plurality of subscriber stations, comprising at least two data transmitters having their output terminals connected to one and the same data bus which comprises two leads, and each having a control input connected to a control arrangement and being controllable by such control arrangement that, at most, one of the data transmitters is activated at a given time, whereby, in their activated condition, the data transmitters respectively output a voltage of a first polarity or of a second polarity to the data bus dependent on the state of a data signal applied to their input, whereby the data transmitters have a higher internal resistance at the output in the passive condition than in the active condition, and whereby at least one data receiver is connected to the data bus, the data receiver assuming, in the manner of a differential amplifier, one of two prescribed signal states at its output given an existing input voltage and dependent on its polarity.
2. Description of the Prior Art
An arrangement of the type set forth above is generally known from the periodical "Elektronik", 6/23 March 1984, page 24 and 12/15 June 1984, pp. 97 and 98.
The known circuit arrangement contains interface modules for serial transmission, each of which is composed of a data transmitter and a data receiver and is suitable for full-duplex or half-duplex operation. In half-duplex operation, the output of the data transmitter is directly connected to the input of the data receiver in every subscriber station. The subscriber stations which can be a computer and peripheral devices connected thereto, for example, are connected to a data bus. The data transmitter and the data receiver in each subscriber station can be driven into an active condition or into a passive condition by control signals.
Since the transmitters, in their active condition, represent a low-impedance voltage source at their outputs, whose voltage is dependent on the logic level of the data signal applied to the respective input, only one data transmitted can be activated at a given time.
To this end, a control signal that drives the data transmitter of a subscriber station into its active condition can be transmitted to all other subscribers and can be used at such other subscribers to make an activation impossible for the time in which the data bus is occupied. It is thereby obvious to transmit the control signal via control lines or via a control bus that is provided in addition to the data bus.